The invention relates to a method of and a forging device for forging crankshafts especially in presses.
In a known method of forging crankshafts in presses a bar is clamped by clamps located at a certain distance and a bar portion between these clamps is upset and simultaneously bent and joggled radially to the bar axis. At the beginning of the working stroke the upsetting velocity is higher than the bending velocity whereas at the end of the working stroke the upsetting velocity is lower then the joggling one.
This known method is used for forging a crank throw so that two crank webs and a crank pin between them are forged during one working stroke. The devices for forging crankshafts by the above said method are known for example from Polish patent specification Nos. 468874, 50720 and 50253 and the U.S. Pat. No. 3,348,407.
Similar devices for forging crankshafts by the method of simultanous upsetting and bending a bar are also known from the Polish patent specification Nos. 112203 and 122409 and the U.S. Pat. No. 4,272,979. These devices are designed to be mounted on a free forging hydraulic press. Each of these devices consists of a base and a head which is designed for reciprocal movement in derection to the base. Between the base and the head the devices are provided with two pairs of movable and mutually coupled die holders which are guided perpendicularly or at an oblique angle to the direction of the head movement. Each of the movable pairs of the die holders is connected with the base by two articulated connectors.
In the said die holders die inserts are fixed for shaping crank webs, crank pins and bearing journals. Between face surfaces of these die inserts there is a bending tool rigidly fixed to the base and an anvil rigidly fixed to the head. During the working stroke, the vertical movement of the head is transformed by the articulated connectors into a horizontal movement of the split die inserts and the holders toward each other, while a bar portion situated between them undergoes upsetting. The horizontal upsetting force which appears in this device can be aproximatively calculated from the formula EQU P.sub.h.sbsb.1 =0.5N.sub.p (cot .alpha.-.mu.)
where
N.sub.p --press capacity, PA1 .alpha.--inclination angle of articulated connectors, PA1 .mu.--coefficient of friction between sliding elements of the device.
The upset material is bent simultaneously toward the anvil by the bending tool. During the working stroke of the device the bending velocity increases in relation to the upsetting velocity to such extent that at the final phase of operation the bending turns into joggling of crank pin material in relation to the material of two adjacent main bearing journals. In this final operation phase the crank throw webs are formed.
This known method and devices could not be applied for forging of long stroke crank throws, particularly for high-power marine engines, because the relation of the bar length which is to be upset between the upsetting dies to the bar diameter would be too high.
The length of the bar which is upset depends on the crank throw volume i.e. volumes of two crank webs and a crank pin. Exceeding the upsetting ratio during the upsetting process causes incorrect forming of the crank webs and also irregular grain flow of the crank throw.
It can be assumed that the biggest free forging presses have capacities from 80 to 120 MN. By means of these presses and the known forging methods and devices solid crankshafts with crank throw stroke up to about 1200 mm can be forged. This limitation results both from the capacity of the press and working space (distance between the press crosshead and the press table, distance between columns, stroke of the press crosshead). However, the biggest crankshafts applied in marine Diesel engines have the stroke up to 2925 mm, so there is not any possibility to forge crankshafts with the stroke larger than 1200 mm. These types of long stroke crankshafts are produced up to now as semi-built up crankshafts assembled by shrinking.
Solid crankshafts have many advantages in comparision with semi-built up crankshafts. Smaller overall dimensions of solid crankshafts allows diminishing the dimensions and weight of engines. Designers and marine Diesel engine producers have undertaken experiments to introduce crankshafts assembled by welding instead of semi-built up crankshafts. However, the welding process is very expensive and time consuming and also demands testing of the welds.